A short-beaked echidna’s penis is usually kept inside its body. When visible, its tip has four bright-red, rosette-like glans—often called “heads”—at the end of a single organ. During an erection, only two glans become erect at a time, and the active pair appears to alternate. The penis is used for mating, not urination.
What does it look like at rest and when erect?
When it is not in use, the short-beaked echidna’s penis is stored internally. The striking image shared by Australian Geographic shows the organ outside the body; the magazine described it as bright red and credited the image to reproductive biologist Jane Fenelon in its 24 May 2021 article.
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At the tip are four rosette-like glans. “Four-headed” is a memorable shorthand, but the more precise description is four glans on one penis—not four separate organs. During an erection, only two of the four become erect at once. Earlier observations indicate that the active pair can alternate from one side to the other.
How does the four-glans anatomy work?
A 2021 anatomical study of the short-beaked echidna, Tachyglossus aculeatus, examined how the organ’s internal structures relate to its one-sided action. The researchers used microCT with iodine staining and microscopy to reveal soft tissues. Their findings are described in a University of Melbourne explainer published 9 June 2021.
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The urethra begins as one tube and branches toward the glans. The researchers also described separate erectile tissues and a branching blood supply. They proposed that blood flow down one side may control which half becomes erect and which urethral branch remains open. This offers a possible anatomical explanation for the two-at-a-time pattern; it does not settle why the arrangement evolved.
Is it used for urination?
No. The penis is used for mating, not for passing urine. Australian Geographic quoted Fenelon saying, “We’re not really sure why it looks so weird but we do know that they only use their penis for mating, not urine.” Its 21 August 2026 explainer also reiterates the distinction between reproductive and urinary functions.
Why do echidnas use only two glans?
The reason is not known. The 2021 researchers raised male competition as a possible factor, but did not establish it as the explanation. The observed alternation and the proposed blood-flow mechanism describe what happens and how it might work—not a proven evolutionary purpose.
What was the “viral” echidna image?
PerthNow reported on 29 April 2022 that Australian Geographic had shared the image on Facebook the preceding Thursday, drawing attention to its earlier coverage. That report documents the share and the viral framing, but does not give a view count or other measure of reach. See PerthNow’s report.
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What the study can—and cannot—tell us
The detailed anatomical account concerns the short-beaked echidna; it should not automatically be applied to every echidna species. The University of Melbourne authors say the work involved echidnas brought injured to Currumbin Wildlife Hospital that could not recover, as well as observations of a tame echidna. The explainer also reports that about 50 injured echidnas are brought to the hospital each year. That is an approximate annual hospital intake, not a population estimate or the number of animals in the study.
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